The present invention relates to an apparatus for purifying the exhaust gas of an engine by means of a catalyst.
While various apparatus are conventionally used to purify the exhaust gas of an engine by means of a catalyst, there is available a model in which heated air is introduced at a point upstream of the catalyst to insure that the catalyst temperature is elevated as fast as possible after engine start-up, thereby enhancing the efficiency of purification. If heated air is introduced into the exhaust pipe right after the engine is started and when the catalyst temperature and, hence, the purification efficiency are still low, the introduced hot air will heat the catalyst and, at the same time, the reaction involving the oxidation of HC and CO in the exhaust gas is accelerated by the catalyst, thereby causing a rapid increase in the temperature of the catalyst.
An exhaust gas purifier of the type under consideration is described below with reference to FIGS. 5 and 6 and is disclosed in copending application Ser. No. 08/033,676. FIG. 5 shows schematically the layout of the apparatus for purifying the exhaust gas of an engine, and FIG. 6 is a graph showing the flow quantity of air that is introduced into the exhaust pipe.
In FIG. 5, reference numeral 1 denotes the engine, 2 is the transmission of engine 1, and 3 is a suction pipe. The portion upstream of the suction path formed by suction pipe 3 communicates with the atmosphere via air cleaner 4.
Shown by 5 is a throttle valve that controls the quantity of air to be sucked into engine 1. Engine 1 is so adapted that it is supplied with a fuel by means of an injector (not shown). Shown by 6 is the exhaust pipe and catalyst 7 for purifying the exhaust gas by a chemical reaction is provided downstream of the exhaust pipe 6.
Shown by 8 is an air pump for introducing air into the exhaust pipe 6. This air pump 8 is of a mechanical type that is driven by engine 1 and the air discharge port of the pump communicates with the exhaust pipe 6 at the portion upstream of the catalyst 7 via an upstream air introducing pipe 9, a control valve 10, a check valve 11, a heater 12, a downstream air introducing pipe 13, etc. The air suction port of pump 8 either opens to the atmosphere or communicates with the suction path.
The control valve 10 is so adapted that it will open and close the air channel communicating with the air discharge port of air pump 8 and the opening or closing action of the valve 10 is controlled by a control unit 14 to be described later. Check valve 11 is so adapted that it will permit the selective passage of air from air pump 8 toward exhaust pipe 6, thereby preventing the exhaust gas from leaking out of the exhaust pipe 6 to enter the air pump 8. The heater 12 is fitted with a heating element (not shown) that generates heat when an electric current is applied and it is so adapted as to heat the air flowing through the above-mentioned air channel. The application of an electric current through the heater 12 is controlled by the control unit 14 as is the control valve 10.
The control unit 14 is so adapted that when the start switch (not shown) for engine 1 is turned on, it will open the control valve 10 while causing an electric current to be applied to the heater 12.
Being thus constructed, the exhaust gas purifier of the present invention is operated in the following manner. When engine 1 is started, air pump 8 starts to work and, at the same time, control valve 10 opens, whereupon air is ejected from air pump 8 and flows through upstream air introducing pipe 9, control valve 10, check valve 11, heating device 12 and downstream air introducing pipe 13 to be introduced into the exhaust pipe 6 at the point upstream of the catalyst 7. The change in the quantity of air thus introduced is shown in FIG. 6. Symbol A in FIG. 6 denotes the time of engine start-up. As shown, the quantity of air introduced into the exhaust pipe 6 is generally constant after engine start-up if the engine is running at constant rpm.
If engine 1 is started, control unit 14 will cause an electric current to flow through the heater 12, so the air discharged from the air pump 8 is heated with the heater 12 before it is introduced into the exhaust pipe 6.
The heated air thus introduced into the exhaust pipe 6 is mixed with the exhaust gas in the exhaust pipe 6 and the resulting mixture flows into the catalyst 7. When both the exhaust gas and the heated air flow into the catalyst 7, the latter is heated by the hot air while, at the same time, HC and CO in the exhaust gas are converted to H.sub.2 O and CO.sub.2 by O.sub.2 in the heated air. In order words, the heat of the hot air and the heat of reaction are effectively used to raise the temperature of catalyst 7 as soon as the engine is started.
The problem with the use of the exhaust gas purifier having the construction described above is that the efficiency of exhaust gas purification decreases as the vehicle continues to run. This is because the wind blowing against the running vehicle cools the heater 12, downstream air introducing pipe 13, etc., thereby reducing the temperature of the heated air to be introduced into the exhaust pipe 6.